1. Field of the Invention
Embodiments of the present invention relate to systems and methods for spectral imaging. More particularly, embodiments of the present invention relate to spectral imaging systems and methods in which a transmission mask and a pair of match dispersive elements are used to produce a spatio-spectral response within a single time step.
2. Background Information
Traditional digital imaging techniques produce images with scalar values associated with each spatial pixel location in imaging spectroscopy, these scalar values are replaced with a vector containing the spectrum spectral information from that spatial location. The resulting datacube is, therefore, three-dimensional (two spatial dimensions and one spectral dimension).
Spectral imaging has traditionally focused on environmental remote sensing and military target recognition tasks. In recent years, however, there has been a dramatic growth in biophotonics applications, and with that growth has come an increased interest in spectral imaging for biological applications (especially those with security applications).
Straightforward application of traditional spectroscopic techniques to spectral imaging, however, can be problematic. The simplest type of spectral imager combines a tomographic (rotational scanning) or pushbroom (linear scanning) front-end with a traditional slit-based dispersive spectrometer. Unfortunately, the sources tend to be weak and spatially-incoherent. Slit-aperture dispersive spectrometers have extremely poor photon collection efficiency for incoherent sources. When the source is also weak, the absolute number of collected photons can be very small. Further, this small number of photons must be apportioned amongst the large number of “cells” in the data cube. As a result, a given spatio-spectral element tends to contain very few photons and hence has a poor signal-to-noise ratio (SNR).
There have been a number of proposed solutions to the light collection problem over the years. Two very advanced solutions are the scanning-Michelson Fourier-transform spectrometers, and multiplexed pushbroom designs based on digital micro-mirror (DMM) technology. Both approaches have proven successful, however they involve expensive components that are not robust.
More robust and inexpensive solutions also exist. The spectral imaging community has developed a number of different direct-view designs that maximize the light gathering efficiency of the systems. These systems do away with the spectrometer slit altogether and simply view the source through a rotating dispersive element. In this approach, the measurements taken at different rotation angles of the dispersive element are projective measurements through the data cube and can be topographically reconstructed. While the photon efficiency of this type of approach is quite high, there is a drawback. The geometry of the system necessarily limits the range of angles over which projections are made. As a result of the Fourier-slice theorem, this yields an unsampled region of Fourier space. Consequently, the estimate of the data cube is inexact. In the tomographic community, this Fourier undersampling is known as the missing cone problem, because the unsampled region is a conical volume in Fourier space. There has been significant work on algorithmic approaches for “filling in” this missing information. The most successful has been the method of projection onto convex sets (POCS).
A coded-aperture based system, which is similar to the direct-view methods in that it is inexpensive, mechanically robust, and has high light-collection efficiency, is described in U.S. patent application Ser. No. 11/580,925 filed on Oct. 16, 2006, which is herein incorporated by reference in its entirety. This coded-aperture based system, unlike the direct-view methods, has no missing cone. The basic design of the system is based on a 2D coded aperture static multimode multiplex spectrometer (MMS). A static MMS is described in U.S. Pat. No. 7,092,101, which is herein incorporated by reference in its entirety. A 2D coded aperture static MMS is described in U.S. patent application Ser. No. 11/334,546 filed Jan. 17, 2006, which is herein incorporated by reference in its entirety.
In view of the foregoing, it can be appreciated that a substantial need exists for systems and methods that can advantageously perform spectral imaging with a high optical efficiency, with a low component and design cost, and without the missing cone problem.